This invention relates in general to apparatus for electrically transferring marking particle images from a dielectric support, and more particularly to a roller transfer apparatus for effecting marking particle transfer, such apparatus having an extended transfer nip exhibiting low pressure between the dielectric support and the roller transfer apparatus.
In typical electrographic reproduction apparatus, marking particles are attracted to a latent image charge pattern formed on a dielectric support to develop such image on the support. The dielectric support is then brought into contact with a receiver member and an electric field applied to transfer the marking particle developed image to the receiver member from the dielectric support. After transfer, the receiver member bearing the transferred image is transported away from the dielectric support and the image is fixed to the receiver member by heat and/or pressure to form a permanent reproduction thereon.
Application of the electric field to effect marking particle transfer is generally accomplished by ion emission from a corona charger onto the receiver member, or by supporting the receiver member on an electrically biased roller holding the member against the dielectric support. While roller transfer apparatus are inherently more complex than corona charger transfer apparatus, roller transfer apparatus offer certain advantages. For example, roller transfer apparatus typically require a lower energy budget, and also maintain a more positive (physical) control over receiver members particularly where a member must be recirculated to have multiple images transferred thereto such as in making multi-color reproductions.
A well known alternate method for electrographic reproduction involves the use of a transfer intermediate. According to this method, a marking particle developed image on the dielectric support is transferred by an electric field to an intermediate member (roller or web) and thereafter transferred by an electric field to a receiver member at a location remote from the zone of transfer of the image to the intermediate member. This method is suitable for sequential transfer of a plurality of images, in register, to the intermediate member prior to transfer of any of the images from the intermediate member to the receiver member. Thus, the plurality of images can be transferred to the receiver member in one step to assure that their relative registration is maintained. Of course, for efficient operation, this method requires more effective cleaning of the intermediate member between transfer to the receiver member.
In the use of roller transfer apparatus, image defects, generally referred to as "halo", "hollow character", and "image disruption" have been found to occur particularly when using smooth paper as the receiver member. Studies have indicated that the strength of the transfer field, receiver member characteristics, and marking particle size all have an impact on the production of such defects. Additionally, the "hollow character" defect is due to the adhesion of the marking particles to each other and to the dielectric support when the receiver member is compressed with the dielectric support by the transfer roller. That is to say, the "hollow character" defect is related, at least in part, to the pressure in the transfer nip, and a substantial reduction in this defect is found to occur when the nip pressure is reduced. For example, U.S. Pat. No. 3,942,888 (issued Mar. 9, 1976, in the name of Maksymiak et al), in addressing the "hollow character" defect, discloses a transfer roller apparatus configured to function at a nip pressure below 6.9.times.10.sup.3 Pa in order to reduce such defect. However, the roller disclosed in this patent has a complex construction including enlarged end portions which serve to provide a gap between the lesser diameter central portion and the dielectric member. As is apparent, a transfer roller of this construction must be capable of being selectively shimmed to successfully function to maintain desired pressure on a variety of receiver member thicknesses.